Flow modulation for facilitating detector ignition

ABSTRACT

Method and apparatus for the analysis of one or more analytes present in a sample carried in a first fluid. The first fluid is combined with a first detector fluid and a second detector fluid to provide a fluid mixture which flows across the surface of an igniter. The analytes are ionized by means of an ionization process. The ion current is collected and measured at a collector electrode adjacent to the igniter. The flow of at least one of the first detector fluid and second detector fluid is modulated during an ignition sequence according to predetermined criteria so as to facilitate flame ignition.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 08/376,717 filed on Jan.23, 1995, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to gas chromatography, and inparticular to the operation of a flame-based detector.

BACKGROUND OF THE INVENTION

Flame-based detectors are used in the field of chromatography for thedetection of specific constituent components (i.e., analytes) of asample that are present in a fluid stream. Two such detectors are theflame ionization detector (FID) and the flame photometric detector(FPD).

Flame ionization detectors operate by burning the material to beanalyzed so as to form ions. When used with a gas chromatograph, thegases eluting from a separation column are mixed with a flammable gassuch as hydrogen and passed through a jet. An energy source, such as anelectrical heating current carried by a resistive heating wire embeddedin the igniter, heats the fluid stream to an ignition condition,whereupon a flame is established. Air is introduced at the periphery ofthe jet so as to form a stable flame when the jet is ignited. Acollector tube is positioned adjacent the jet, and an electrometer isconnected in a circuit with the collector and the jet so as to collectthe ions formed in the flame. A current is produced for measurement bythe electrometer in proportion to the rate at which the ions arecollected.

Although the FID ignition depends upon an adequate flow of air, theflame will not ignite at normal operating air flow rates, and thereforeit is known to provide a fixed reduction in the air flow (typically,about 70%-80%) received by the detector so as to encourage ignition. Oneprior art thermionic detection technique therefore attempts to divertthe air stream from the detector during the ignition of the flame. Oneprior art flame detector utilizes an "air dump" valve which is manuallyactivated by the operator. The air dump diverts part of the air flow tothe atmosphere when the ignition is attempted.

The flame photometric detector is ignited in a fashion similar to thatof the flame ionization detector. However, because an FPD uses a flamethat is substantially enriched with hydrogen rather than air, ignitionin an FPD is typically aided by increasing the air flow, while thehydrogen flow is held constant, to prevent an explosion when the flameis ignited. Some prior art thermionic detection techniques thereforeattempt to increase the air stream delivered to an FPD when an ignitionis attempted.

However, the aforementioned approaches have significant drawbacks. Theaddition of a valve to divert or increase air flow is also more costlyand complex to implement than is desirable. The practice of diverting orincreasing one or more of the gas flows is typically controlled by anoperator by manual intervention, which is cumbersome, inconvenient, andsubject to error. Apparatus for providing automation of the diversion ofa gas flow has been disclosed; see, for example, U.S. Pat. No.4,346,055.

Even repeated attempts at ignition (or re-ignition) can be unsuccessfulat times when the operating conditions of the chromatograph aremarginally suitable, or unsuitable, for ignition. Furthermore, theoperator is typically unaware of the optimal conditions that arerequired for ignition, thus compounding the difficulty. The operatortypically must make repeated attempts at ignition before successfulignition occurs, and does so without knowledge of the optimal theconditions for ignition, and therefore the ignition procedure can besignificantly longer and more arduous than is acceptable. Thesedrawbacks are also found in the aforementioned automated flow diversionapparatus.

The aforementioned drawbacks are even more bothersome during an attemptto re-ignite a detector during a flame-out condition that has arisenduring an ongoing operation of the chromatograph. For example, the lossof a flame during an analytical run requires an immediate reignition inorder to achieve a quick resumption of the operation of the detector.Otherwise, the results from a significant amount of the analytical runwill be compromised.

SUMMARY OF THE INVENTION

This invention provides a method and apparatus for an improvement in theflame-based detection of one or more analytes in a sample.

In a first preferred embodiment of the present invention, an analyticalinstrument for detecting the presence of an analyte in a first fluid maybe constructed to include a pneumatic manifold for providing aselectable flow of at least one of a plurality of fluids, said pluralityincluding the first fluid, a first detector fluid, and a second detectorfluid; a pneumatic controller, responsive to a control signal, forcausing said selectable flow to be modulated; a flame-based detector,operably connected to the pneumatic manifold for receiving the modulatedselectable flow; and a programmable computer for providing said controlsignal in a predetermined ignition sequence so as to effect saidmodulation according to predetermined flow modulation criteria thatfacilitate ignition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is simplified schematic representation of a chromatographconstructed according to the present invention.

FIG. 2 is a simplified cross-sectional illustration of a portion of thechromatograph of FIG. 1, showing a first configuration of a pneumaticcontrol section and a flame-based detector constructed according to thepresent invention.

FIG. 3 is a simplified cross-sectional illustration of the portion ofthe chromatograph of FIG. 1, showing a second configuration of thepneumatic control section.

FIGS. 4 and 5 are representations of successive steps that are employedin an ignition sequence provided in the chromatograph of FIG. 1.

FIGS. 6-9 are graphical representations of successful conditions forestablishing ignition in an experimental test of a flame-based detectoroperated according to the embodiment in FIG. 3, in accordance with theoperation and benefit of an inventive feature of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus and methods of the present invention may be employed inparticular to improve the detection of an analyte that may be present ina variety of fluids. Gases are the preferred fluids according to thepractice of the present invention, and therefore the followingdescription of the invention will include a description of thearrangement, construction, and operation of pneumatic devices, and isdirected to the control of a plurality of gaseous streams in a detectorin a gas chromatographic analytical system (hereinafter, achromatograph). However, for the purposes of the following description,the term "pneumatic" will also be considered to refer to all types offluids.

Further examples that are particularly benefited by use of the presentinvention include supercritical fluid chromatography and high-pressuregas chromatography (HPGC). However, it should be understood that theteachings herein are applicable to other analytical instruments,including liquid chromatographs, high-pressure liquid chromatographs(HPLC), clinical analyzers, flow-injection analyzers, laboratory waterpurification systems, syringe-type reagent dispensers, manual andautomated solid phase extraction (SPE) instruments, supercritical fluidextraction (SCF) instruments, stopped-flow spectrophotometers, automatedprotein or nucleic acid sequencers, and solid phase protein or nucleicacid synthesizers.

A new and novel analytical instrument is shown in FIG. 1 and isgenerally designated chromatograph 10. In the preferred embodiment, thechromatograph 10 is a Hewlett-Packard HP6890 gas chromatograph. In orderto perform a chromatographic separation of a given sample compound, asample is injected with a pressurized carrier gas by means of aninjector 12. The carrier gas supplied to injector 12 is provided from asource 12A through one or more pneumatic manifold assemblies 13, each ofwhich serves in part to control and redirect a plurality of gas flows,including the carrier gas and a plurality of detector gasses ofappropriate types, such as air, hydrogen, and make-up gas. The detectorgases are provided from respective sources (one such source 24A isshown) to the pneumatic manifold assembly 13. Suitable fluid-handlingdevices such as valves, sensors and the like in the pneumatic manifoldassembly 13 are operated under the control of the computer 22 by way ofcontrol signals provided on a data and control lines 28, 30. Forexample, the pneumatic controller 26 effects control of, among otherthings, fluid flow rate, fluid pressure, fluid flow regulation, and thecontinuity or discontinuity of flow. As further example, the time duringwhich a particular valve in the pneumatic manifold assembly 13 willremain open and closed in relation to control signals received on thedata and control line 28 and in accordance with certain operatingconditions of the chromatograph 10. The control and data line 30 alsoallows the rerun of sense information from suitable sensors andsignal-interface electronics that are provided in the pneumatic manifoldassembly 13. Accordingly, the computer 22, pneumatic controller 26, andpneumatic manifold 13 may be operated to effect a modulation of any ofthe aforementioned gas flows, either individually or in combination.

A column 14 is positioned within an oven 16. The carrier gas/samplecombination passing through column 14 is exposed to a temperatureprofile resulting in part from the operation of a heater 18 within oven16. During this profile of changing temperatures, the sample willseparate into its components primarily due to differences in theinteraction of each component with the column 14 at a given temperature.As the separated components exit the column 14, they are detected by thedetector 24.

As the components exit column 14 they are detected by a flame-baseddetector (hereinafter, detector) 24. In particular, and in accordancewith a feature of the present invention, the pneumatic controller 26modulates the flow of one or more of the detector gases that areprovided to the detector 24 during an ignition sequence, as will bedescribed below.

Computer 22 maintains overall control of all systems associated with gaschromatograph 10. It will be recognized that any particular gaschromatograph may include more systems than those described in relationto the present invention. It will also be understood that althoughcomputer 22 is shown as a single block, such computer includes a centralprocessing unit and all associated peripheral devices, such as randomaccess memories, read-only memories, input/output isolation devices,clocks and other related electronic components. In the preferredembodiment, the central processor used in computer 22 is amicroprocessor. As such, computer 22 includes a memory in whichinformation and programming can be stored and retrieved by knownmethods. However, it will be appreciated that the programmed control ofpneumatic controller 26 can be implemented by other computing means,such as an embedded microprocessor or dedicated controller circuitincorporated in the pneumatic controller 26. Also, the programmingassociated with computer 22 that is utilized in relation to the presentinvention will be readily understood from the description herein.

An electronic control panel 50 is shown to include at least two maininput/output components, namely a keypad 58, and a display 60. Bymonitoring the operation of the chromatograph 10 by signals from certaincomponents, such as the detector 24, the computer 22 can initiate andmaintain certain functions required for an analytical run. Consequently,indicating or prompt messages can be generated by computer 22 anddisplayed on display 60. Operating commands and other information areentered into computer 22 by way of keypad 58. One particular data typeis detector ignition setpoint values and one particular operatingcommand is a detector ignition command, both of which may be prompted bymessages displayed on display 60 and the requisite command or data areentered through keypad 58. Another particular type of data is a detectorignition lit offset value, which may be acknowledge by messagesdisplayed on display 60 and modified by commands and data enteredthrough keypad 58. The ensuing ignition sequence which relates to thepresent invention is then automatically provided under control of thecomputer 22 as described below in reference to FIGS. 4 and 5.

The control of one or more fluid flow characteristics is provided asshown in FIGS. 2 and 3. In the embodiments illustrated in FIGS. 2 and 3,the computer 22 controls the flow of the make-up fluid, the first fluid,the first detector fluid, and the second detector fluid by transmittingan appropriate signal to the pneumatic controller 26, which in turnprovides respective signals to a respective valve in the pneumaticmanifold assembly 13 to increase or decrease the amount of fluid flowingtherethrough to the detector 201. In particular, the fluid flow controlin the embodiment illustrated in FIG. 3 is preferably provided viaelectronic pneumatic control (EPC). For further details of electronicpneumatic control techniques, one may consult, for example, Klein, etal., U.S. Pat. No. 4,994,096 and U.S. Pat. No. 5,108,466, thedisclosures of which are incorporated herein by reference.

FIG. 2 shows a schematic illustration of a first preferred embodiment ofa detector 201 preferably constructed as a FID and a pneumatic controlsection 202 that is best suited for operation in a non-EPCconfiguration. The detector 201 constructed to include an igniter 211,an ignition line 212, and a collector electrode 213. The igniter 211 andthe collector electrode 213 are aligned in the interior of a jet 214that is mounted in a passageway defined by a fluid-directing structure210. An electronic power supply (not shown) provides a controlled amountof electrical current or voltage on the ignition line 212 to cause aselectable amount of heat in the igniter 211. In response, the fluidflow in the proximity of a jet 214 achieves an elevated temperature andis ignited. A vent tube 232 allows the combustion and further passage ofthe fluid mixture from the detector 201. The collector electrode 213 iselectrically connected to an ion current measurement device (not shown)such as an electrometer which is used to measure the magnitude ofionization current that flows from the jet 214 to the collectorelectrode 213. The resulting ion current is measured to provide achromatogram.

A fluid mixing structure 222 communicates with the fluid-directingstructure 210 for directing the following fluids toward the igniter 211:a first fluid supplied on a first fluid supply line 224, a make-up fluidsupplied on a make-up fluid line 225, a first detector fluid supplied ona first detector fluid line 226, and a second detector fluid supplied ona second detector fluid line 227, preferably, the first fluid line 224is integral with the column 14 and hence the first fluid comprises aheated, gaseous combination (under pressure) of the sample that is to beanalyzed and a carrier gas. The makeup fluid also preferably comprisescarrier gas; the first detector fluid comprises pressurized hydrogen(H₂) gas; and the second detector fluid comprises air at ambientpressure and temperature. The make-up fluid and the first detector fluidare combined via a conduit 229 connected between the fluid mixingstructure 222, the make-up fluid line 225, and the first detector fluidline 226. Also included are a make-up fluid pressure regulator 235, amake-up fluid valve 225V, and a make-up fluid restrictor 225R; firstdetector fluid valve 226V and restrictor 226R; and second detector fluidvalve 227V and restrictor 227R. In the instance that the detector 201 isconstructed as a FID, it is contemplated that a predetermined pneumaticvolume is provided in the second detector fluid line 227 between thevalve 227V and the restrictor 227R. (In the alternative instance thatthe detector 201 is constructed as a FPD, it is contemplated that apredetermined pneumatic volume would be provided in the first detectorfluid line 226 between the valve 226V and the restrictor 226R.) Thevalves 225V, 226V, and 227V are preferably solenoid valves that aresubject to the control of the pneumatic controller 26 as will bedescribed in greater detail below.

FIG. 3 illustrates an alternative embodiment 202A of the pneumaticcontrol section 201 of FIG. 2 that is best suited for operation as anEPC configuration. That is, in FIG. 3, the valves 225V, 226V, and 227Vare preferably provided in the form of proportional valves that aresubject to the control of the pneumatic controller 26 according tosignals received by the computer 22 from sensors 225S, 226S, and 227S,as will be described in greater detail below. Preferably, such sensorsare pressure sensors that provide sense signals indicative of therespective pressures in the make-up fluid line 225, first detector fluidline 226, and second detector fluid line 227.

In the embodiment illustrated in FIG. 3, sensors 225S, 226S, 227S eachsense a particular fluid parameter, such as fluid pressure or fluidflow, and transmits a feedback signal representative of such parameterto the computer 22. By monitoring the sense signals from sensors 225S,226S, 227S, the computer 22 can effect near-instantaneous alteration ofthe flow of each fluid that is provided to the detector 201 at anydesired time.

In the preferred embodiment of the computer 22, the procedures necessaryto set up or operate chromatograph 10, so that a particular gaschromatographic separation test or analytical run can be conducted, areautomated. The contemplated automation allows the operator to programevents using programming via a table of fluid flow setpoints, a runtable, and by clock time programming. A plurality of timed events may beprogrammed in each run table for execution during an analytical run. Runtime programming allows certain setpoints to change automatically duringa run as a function of the chromatographic run time. For example, anevent such as detector ignition may be programmed to occur prior toinjection. Such programming is contemplated as being applicable to theoperation of the pneumatic controller, and particularly to the controlof at least one of the first detector fluid flow and the second detectorfluid flow.

Certain programmed steps effected by computer 22 in controlling theoperation of the pneumatic controller 26, which relate to and are inaccordance with the present invention, are illustrated in FIGS. 4 and 5,In the preferred embodiment, the operator may enter data regarding theoperation of the pneumatic controller 26 into the computer 22 by use ofthe keypad 58. The computer 22 operates to store the entered informationinto memory. The data thus entered may include one or more commands thatare to be implemented immediately, or if necessary, the entered data maybe stored in the form of one or more tables for later access. Forexample, the programmed events may be arranged in order of executiontime in a run table. Text denoting the characteristics of each event maybe displayed on the display 60.

In the embodiments illustrated in FIGS. 2 and 3, individual fluidstreams combine to form a fluid mixture that is restricted to pass theigniter 211 and the collector electrode 213. The flow characteristicsand the composition of the fluid mixture that passes the igniter 211will determine the success or failure of the ignition mechanism thatoccurs at the igniter 211. Hence, in a departure from the prior art, thecontent of the fluid mixture is temporarily altered during certain stepsin an ignition sequence, so as to favor the ignition mechanism andthereby facilitate ignition. Specifically, the flow of at least one ofthe fluid streams is modulated to effect either an increase or decreasein the fluid flow.

Accordingly, and in a particular feature of the present invention, aparticular modulation of the flow of the first or second detector fluidduring an ignition sequence has been found to facilitate detectorignition. Such ignition has been successful in instances that mayotherwise be unsuccessful due to the influence of the content and othercharacteristics of the fluid mixture. In the preferred embodiment, andas illustrated in FIGS. 4 and 5, either the air flow rate or thehydrogen flow rate is modulated during a portion of the ignitionsequence. The implementation of the modulation will differ slightlyaccording to whether the embodiments of FIG. 2 or FIG. 3 are provided.Because the flow through the solenoid valves shown in the configurationin FIG. 2 can only be turned on or off, rather than varied continuously,a modulated flow is preferably achieved by cycling the appropriate valve(valve 227V for FID, valve 226V for FPD) on and off, starting at a lowduty cycle and increasing it (preferably at a fixed frequency) untilfull flow is attained. In the configuration illustrated in FIG. 3, thecontemplated EPC achieves a modulated flow by opening the appropriatevalve (valve 227V for FID, valve 226V for FPD) in a continuous ramp,starting at a low value and increasing it until full flow is attained.

In an instance of non-EPC control, the second detector fluid flow isbest modulated by a solenoid valve according to the practice of:

a) fixing the flow duty cycle and varying the frequency of modulation;

b) fixing the frequency of modulation and varying the flow duty cycle;or

c) varying both the flow duty cycle and the frequency of modulation.Preferably, a fixed frequency option is selected because it is theeasiest to implement in firmware resident in the computer 22. Thepreferred frequency of modulation is chosen to offer:

d) for all operating conditions, a predetermined range of duty cyclesthat will result in ignition, and

e) for the most severe conditions (such as may be found during a veryhigh second detector fluid flow and when helium is used as the make-upgas), the greatest span of duty cycles which will result in ignition.

In another feature of the present invention, the aforementioned criteriamay be applied to a particular chromatograph 10 such that a modulationenvelope may be predetermined to provide the requisite flow modulationthat will facilitate, if not ensure, a successful ignition even underworst case conditions.

In a further aspect of the present invention, in the ignition of a FID,the make-up fluid flow is discontinued during a portion of the ignitionsequence and then resumed after the flame is lit.

In a still further aspect of the present invention, in the ignition of aFID, the first fluid flow is discontinued during a portion of theignition sequence and then resumed after the flame is lit.

In the preferred embodiment of the chromatograph 10, detector 24 isprovided as a plurality of separately located detectors, e,g., a frontdetector and a back detector. Also in the preferred embodiment, thefirst detector fluid is provided as hydrogen gas. FIG. 4 illustrates afirst preferred ignition sequence that is pertinent to the operation ofthe chromatograph 10 when one such detector 24 is provided in the formof an FID. In the preferred embodiment, each of the first, second, andthird delay periods are approximately 2-3 seconds. FIG. 5 illustrates asecond preferred ignition sequence that is pertinent to the operation ofthe chromatograph 10 when one such detector 24 is provided in the formof an FPD. In the preferred embodiment, each of the first, second, andthird delay periods are approximately 1-2 seconds.

Experimental Results

The advantages of the above-described embodiments were demonstrated in aseries of ignition repeatability experiments performed on a testapparatus that included a FID mounted in a Hewlett-Packard HP5890 gaschromatograph. Pneumatic control was provided according to the non-EPCconfiguration illustrated in FIG. 2. Consistent ignition was found tooccur according to certain criteria, as will now be described.

One modulation frequency that was found to satisfy these criteria in thechromatograph under test was 0.5 cycles/sec.

A slow duty cycle was found to be preferable because the time demands onthe firmware in the computer 22 are lessened and any differences invalve actuation speed are less likely to affect the success of theignition. Based on the test data, the preferred duty cycle started at10% and was stepped through several duty cycles until at least 60%, atwhich point the relevant valve can remain full-on.

Adding a pneumatic volume between the valve 227V and the frit 227F wasfound to reduce the necessary frequency of actuation and, in addition,increase the range of duty cycling at which the flame will ignite. Avolume of 1500 mm³ was found to be the preferred amount.

FIGS. 6-9 illustrate the test results that indicate the effect ofcertain operating conditions in the chromatograph upon ignition,according to a relationship between flow duty cycle and modulationfrequency. In each instance, a successful ignition was found to occurwithin one or more particular modulation envelopes, each of which relatea range of flow duty cycles and modulation frequencies. The illustratedmodulation envelopes are provided as examples of modulation envelopesthat were particularly successful is achieving consistent ignition inthe FID of the test apparatus, and in no way should be consideredlimiting.

FIG. 6 shows first (A), second (B), and third (C) modulation envelopesrealized in the aforementioned test apparatus according to a variationin pneumatic volume. The response curves (A), (B), and (C) correspond tothe pneumatic volume being implemented as 1500 mm³ , 3000 mm³, and 785mm³ respectively.

FIG. 7 shows a modulation envelope realized in the aforementioned testapparatus according to an interruption in make-up gas during theignition sequence.

FIG. 8 shows first (A) and second (B) modulation envelopes realized inthe aforementioned test apparatus according to differing air flow ratesduring the ignition sequence. The first and second response curves (A)and (B) correspond to the air flow rates being implemented as 400milliliters/minute and 650 milliliters/minute, respectively.

FIG. 9 shows first (A) and second (B) modulation envelopes realized inthe aforementioned test apparatus according to differing jet orficesizes that were used during the ignition sequence. The first and secondresponse curves (A) and (B) correspond to the jet orifice beingimplemented as 0.030 inches and 0,011 inches, respectively. FIG. 9illustrates at least two worst case ignition conditions.

Advantages of the Invention

By satisfying the ignition flow requirements via valve modulation ratherthan diverting fluid flow, the preferred embodiment may be constructedwithout a diverter valve and the associated costs of tubing andfittings, additional assembly labor, and machining of parts. Reliabilityand ease of use are increased.

The presence of make-up gas and/or carrier gas, each of which underminessuccessful ignition by diluting the hydrogen and oxygen in the fluidmixture and by cooling the igniter, may be eliminated during a portionof the ignition sequence.

In contrast to systems constructed according to the prior art, asuccessful ignition in the preferred embodiment is not as subject tomanufacturing production variations. Heretofore, a normal variation in,for example, the gas source pressure, which may stray out of the rangerequired by an operating condition for proper ignition would cause theoperator to experiment with differing source pressures to try to find areliable operating region. In the preferred embodiment, the first orsecond detector fluid flow is modulated during the ignition sequence andtherefore will pass through the optimum fluid mixture that facilitatesignition. The modulation is automatically effected during the ignitionsequence, in a manner not generally noticeable by the operator, therebyensuring a reliable flame ignition.

Although the invention has been described with reference to theabove-described preferred embodiments, variations and modifications arecontemplated as being within the scope and spirit of the presentinvention.

What is claimed is:
 1. An analytical instrument for detecting thepresence of an analyte in a sample fluid, comprising:a pneumaticmanifold, for providing a selectable flow of at least one of a pluralityof detector fluids, said plurality including a first detector fluid anda second detector fluid; a controller, responsive to an electroniccontrol signal, for causing said selectable flow to be modulated; aflame-based detector, operably connected to the pneumatic manifold forreceiving the modulated selectable flow; and a programmable computer forproviding said electronic control signal, said computer including flowmodulation sequence means for determining flow modulation criteria thatfacilitate ignition and signal generating means for providing saidelectronic control signal in a predetermined ignition sequence whereinsaid modulated flow is effected according to a continuous transitionfrom a first predetermined flow rate to a second predetermined flow rateand according to said flow modulation criteria.
 2. The analyticalinstrument of claim 1, wherein the flame-based detector is a flameionization detector, the first detector fluid is hydrogen, and the flowmodulation is applied to the first detector fluid.
 3. The analyticalinstrument of claim 1, wherein the flame-based detector is a flamephotometric detector, the second detector fluid is air, and the flowmodulation is applied to the second detector fluid.
 4. The analyticalinstrument of claim 1, wherein the computer includes a programmabletable, and wherein the predetermined flow modulation criteria areprovided according to the programmable table.
 5. The analyticalinstrument of claim 1, wherein the flame-based detector furthercomprises:a fluid mixing structure for receiving the plurality ofdetector fluids so as to provide a fluid mixture; an igniter capable ofheating of the fluid mixture to produce an ignition condition; acollector electrode for receiving the ion current; and a fluid-directingstructure for aligning the igniter and the collector electrode in aspaced relationship and for directing the fluid mixture so as to contactthe igniter and the collector electrode.
 6. The analytical instrument ofclaim 1, wherein the programmable computer further comprises means forproviding the first and second predetermined flow rates according to apredetermined flow modulation envelope.
 7. The analytical instrument ofclaim 6, wherein the programmable computer further comprises means formodulating the selectable flow according to at least one of a flow dutycycle and a flow modulation frequency.
 8. The analytical instrument ofclaim 1, wherein the programmable computer further comprises means formodulating the selectable flow according to a continuous ramp betweenthe first predetermined flow rate and the second predetermined flowrate.
 9. The analytical instrument of claim 8, wherein the ramprepresents a steady increase of flow the first predetermined flow rateto the second predetermined flow rate.